1. Field of the Invention
The present invention relates to a process for preparing prilled porous ammonium nitrate. More particularly, the present invention relates to an industrially advantageous process for preparing prilled porous ammonium nitrate which has excellent properties as an ammonium nitrate fuel oil explosive (hereinafter abbreviated as "ANFO explosive").
2. Description of the Prior Art
The prilled ammonium nitrate which is used for ANFO explosives and which is different from the prilled ammonium nitrate used for fertilizers must have various properties such that it has high fuel oil absorption. The high fuel oil absorption properties greatly improve the blasting power of ANFO explosives. The prilled nitrate should have an appropriate hardness of 15 to 40 percent so that it does not cake nor powder during handling such as packaging, transportation or the like. However, it should be capable of being powdered under appropriate conditions so that it can be mechanically charged in high density as an ANFO explosive.
In one industrial process for preparing prilled ammonium nitrate for ANFO explosives ammonium nitrate is prilled by spraying an aqueous ammonium nitrate solution of 93 to 98 percent by weight at an elevated temperature of 135.degree. to 160.degree.C from the top of a prilling tower, collecting the prilled ammonium nitrate containing 2 to 6 percent by weight water at the bottom of the tower and further drying it at 90.degree. to 110.degree.C to a water content usually below 0.5 percent by weight, preferably below 0.3 percent. However, it is extremely difficult to carry out the drying step effectively because fine crystals of ammonium nitrate mass on the surface of the prilled ammonium nitrate to form a shell which hinders evaporation of the water present within the particles.
In one method of obtaining a prilled ammonium nitrate which is porous and as high fuel oil absorption properties or which prevents the caking of prilled ammonium nitrate during storage by removing the water content therefrom, a method of utilizing the cracks formed in the ammonium nitrate particles by the transitions of the ammonium nitrate crystal phases has been proposed, for example U.S. Pat. No. 2,382,298 and Japanese Pat. No. 22375/1966. That is, ammonium nitrate possesses the crystalline phases, I, II, III, IV and V and the transition temperatures therebetween are usually as described below. In each transition volume changes occur upon shrinking and expansion which crack the surfaces of the prilled ammonium nitrate particles.
______________________________________ Shrinking Expansion Shrinking Expansion .fwdarw. .fwdarw. .fwdarw. .fwdarw. I II III IV V .rarw. .rarw. .rarw. .rarw. Expansion Shrinking Expansion Shrinking (125.degree.C) (85.degree.C) (32.degree.C) (-18.degree.C) ______________________________________
The method employed in the above mentioned U.S. Pat. No. 2,382,298 involves a rapid quench of prilled ammonium nitrate obtained by prilling a concentrated ammonium nitrate solution containing 4 to 6 percent by weight water to a temperature less than 32.degree.C (III.rarw.IV transition point, at this point the water content of the prilled ammonium nitrate is 3.0 to 5.5 percent by weight) and subsequently raising the temperature to about 100.degree.C to dry the prilled ammonium nitrate. The above mentioned U.S. Patent discloses that prilled ammonium nitrate containing 4 percent by weight water can have its water content lowered to 0.3 percent by weight after quenching by drying it in hot air for 50 minutes. In other words the method involves cracking the surface of prilled ammonium nitrate by raising and lowering the temperature thereof up and down at the III.revreaction.IV transition point (32.degree.C) thereby prompting the removal of water in the particles.
Also the method of Japanese Pat. No. 22375/66 involves cracking the surface of prilled ammonium nitrate by cooling to a temperature less than 32.degree.C and then reheating the cooled material which removes water and simultaneously renders the nitrate particles porous. However, the ammonium nitrate particles obtained by these methods are not only so soft that they are unsuitable for ANFO explosives, but they also are defective in that they cake during handling in packaging and transportation. Other techniques have been proposed for preventing the caking of prilled ammonium nitrate, such as by adding various organic substances as anti-caking agents, because the more complete the prevention of caking, the more difficult it becomes to remove water from ammonium nitrate. However, when an organic substance is added to the ammonium nitrate, it becomes discolored, deteriorates or changes pH since the amount of organic substance added must be large because it is not sufficiently dispersed. Therefore, in order to prevent the caking of ammonium nitrate to such a degree that the defects do not occur and so that only small amounts of organic substance are used, the organic substance needs to be uniformly dispersed in the ammonium nitrate particles. This is usually accomplished by spraying an aqueous solution containing the substance onto the ammonium nitrate before drying the ammonium nitrate. On the other hand, when the prilled ammonium nitrate is dried, powdered dust is unavoidably generated and the yield is usually lowered to 85 to 90 percent. In order to improve the yield, the powdered dust generated in the drying step is recirculated to the ammonium nitrate solution to be prilled for reuse. However, when an organic substance is added to the ammonium nitrate solution before drying as described above, if powdered dust as it is generated is recirculated to the prilling step, the organic substance is exposed to high temperatures such as, for example, 180.degree.C in the prilling tower and stands in danger of exploding since the organic substance is concentrated several times in the powdered dust generated. Therefore, the recovered powdered dust is usually of no use except as raw materials for cheap compounds or mixed fertilizers and thus the method has the extreme disadvantage of being very uneconomical.
A need, therefore, continues to exist for a method of economically prilling ammonium nitrate such that it is sufficient for use in ANFO explosives.